1. Field of the Invention
The invention concerns an arrangement for cooling an electric cable which comprises inner and outer conductors at high-voltage and zero potential, respectively, which are arranged concentrically with each other and are cooled to a low temperature, and which is provided at each of its ends with a cable termination, in which said inner and outer conductors are connected with respective inner and outer ordinary conductors which are situated in the gas stream of an evaporating cooling medium.
2. Description of the Prior Art
In order to economically achieve a.c. transmission of large amounts of electric power by means of a cable with conductors cooled to a low temperature, particularly by means of a superconductor cable, high transmission voltages are necessary. The reason for this is mainly that in order to keep the a.c. losses of the superconductors low, the magnetic induction at the conductor surface must not exceed a given flux density, the so-called "critical" flux density. In order to increase the transmitted power further, however, the conductor surface area and therefore, also the conductor diameter in a conductor arrangement with concentric inner and outer conductors must be increased proportionally with the current.
On the other hand, it is necessary to keep the conductor diameter small, because the thermal losses of the deep-cooled conductor arrangement increase with the surface area because of radiation and thermal inflow via the customary mounting and support devices. Also the relatively high cost of the superconductive material of the conductors and the desire to keep the right-of-way for the cable as narrow as possible, lead to small conductor diameters.
The cooling losses of the cable termination required for such cables are essentially determined by the operating current of the cable, which fixes the cross sections of the ordinary conductors to be optimized in the cable terminations due to their Joule losses and thermal inflow from about 300.degree. K to about 4.degree. K. It is therefore desirable to chose, for a given transmission power, the transmission voltage as high as possible, so that the current can be kept as low as possible.
The maximum operating voltage of a cable and its cable terminations connected with it is determined not only by the attainable dielectric strength of the conductor insulation and its dielectric losses, but also by the conditions of the cooling loops required therefor. In a cooling loop for cooling the superconductors of the cable, one must take into consideration that the inner conductor is at high-voltage potential. Because the coolant cools the superconductors advantageously by direct contact, it likewise assumes this potential. When the coolant for the inner conductor is fed-in, the full voltage must therefore be overcome. Similar requirements also apply to the cable terminations. Their inner ordinary conductors, which are at high-voltage potential, which are connected with the superconductors as well as also with a high-voltage supply lead, are cooled by direct contact with the coolant. This insures good heat transfer and favorable heat removal. These cable terminations are therefore each provided with one coolant supply line each and a corresponding coolant discharge line at room temperature, which must be laid out for the full potential gradient.
The temperature range for cooling the superconductors of the cable is relatively small, becuase the a.c. losses increase with temperature and must in general be limited. For niobium, for instance, a cooling range between 4.2.degree. and 6.degree. K must be provided, while for niobium-tin alloy where the a.c. losses are higher, the cooling range may extend from about 4.5.degree. to 10.degree. K. In contrast thereto, the temperature range for cooling the ordinary conductors of the cable terminations is from about 4.2.degree. K to about 300.degree. K, corresponding to the temperature gradient of the ordinary conductors. Such a temperature transition can be provided, for instance, by a bath with boiling helium, whose evaporating helium rises at the ordinary conductors, which may, for instance, be in the form of wires or laminations, and cools the latter. Such a design is known, for instance, from "The Review of Scientific Instruments", vol. 38, no.12, Dec. 1967 pages 1776 to 1779.
Because of the differences in temperature ranges and the state and pressure conditions, it is advisiable to separate the cooling loops for the cable lines as well as for the cable terminations, in order to obtain an optimum match to the refrigeration machines required in operation. Since, however, good thermal contact exists with the ordinary conductors by means of the leads to the superconductors, it is necessary that these contact points in the cable terminations have the same temperature as the cooling loop for the superconductors of the cable at these points, in order to avoid nonuniformity in the cooling of the superconductor or the ordinary conductors of the cable terminations. Otherwise the cable terminations would be cooled by means of the cooling loop for the superconductors, or vice versa, this cooling loop would be cooled by means of the cable terminations.
It is therefore an object of the invention to provide an arrangement for cooling the cable mentioned at the outset with cable terminations, in which the requirements mentioned are met and which makes economical operation of the cable line possible.